Why am I getting these phone calls ?

Every so often, I get a phone call . When I answer, I say "Hello !", and nobody at the other end says anything . Some of these phone calls come from inside my state, and some of them come from another state . My cell phone has a caller ID feature which lists all my incoming and outgoing phone calls . When... show more Every so often, I get a phone call . When I answer, I say "Hello !", and nobody at the other end says anything . Some of these phone calls come from inside my state, and some of them come from another state . My cell phone has a caller ID feature which lists all my incoming and outgoing phone calls . When I call these numbers back, I usually get a recording that says "The number you are calling is either changed, disconnected, or is no longer in service !" . If that phone number is invalid, how were THEY able to call ME ? If they're not going to say anything, why are they calling me ? Can I put a stop to these phone calls ? Thank you in advance for your answers .
Update: How can they be scams ? Nobody asks me to invest any money in anything !
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